butment

A butment is the solid masonry or concrete structure built at each end of a bridge to receive and distribute the weight of the superstructure into the earth. It performs the critical function of transferring all vertical and horizontal forces from the bridge deck, arches, and traffic loads down into stable ground or rock. Without proper butments, a bridge cannot stand.

The butment differs from a pier, which sits within a stream or span to support intermediate arches or beam sections. Butments anchor the bridge at its terminations. In arch bridges, the butment must resist the outward thrust generated by the arch action itself, which pushes horizontally as well as down. In beam bridges with no arches, the butment simply supports the dead load and live load of the deck. The mass and strength of a butment must be calculated to handle these specific stresses without settlement or failure.

Variants and construction

A simple butment for a short beam bridge may be a single rectangular block of stone or concrete, perhaps 3 to 5 meters deep. Larger bridges, especially historic arch structures, often have butments of substantial thickness and height, sometimes 8 to 12 meters or more, built in layers of ashlar stone with mortared joints or in modern practice, reinforced concrete. The face of a butment is typically vertical or slightly battered (sloped) for stability and appearance. Behind this face lies the backing, which may be rubble masonry or fill.

Failure modes include settlement from poor foundation bearing, frost heave in cold climates if drainage is inadequate, or loss of bond between old masonry courses due to water infiltration and freeze-thaw cycles. Historic stone butments are vulnerable to lime mortar deterioration and moss or lichen growth that traps moisture. Modern inspection focuses on cracks radiating from the arch spring line, which signal movement and thrust concentration.

The term "butment" comes from the verb "but," meaning to strike or abut against. It is the structure against which the arch or beam abuts. The word is most common in British and Commonwealth construction practice; in American usage, the same structure is often called an abutment. Both terms are correct, though abutment has become standard in modern codes and engineering drawings.